c++ - POSIX 线程同步和/或 pthread_create() 参数传递问题

Question

我正在尝试创建许多不同的线程，这些线程需要等待所有线程创建完毕才能执行任何操作。这是一个大程序的一小部分，我只是想逐步进行。当创建每个线程时，它会立即被信号量阻塞。创建所有线程后，我遍历并释放所有线程。然后我希望每个线程都打印出它的线程号以验证它们是否都在等待。我只允许一个线程使用另一个信号量一次打印。

我遇到的问题是，虽然我创建了线程 #1-10，但线程打印出它是 #11。此外，一些线程说它们与另一个线程具有相同的编号。是我传递 threadID 时出错还是同步时出错？

相关代码如下:

//Initialize semaphore to 0. Then each time a thread is spawned it will call
    //semWait() making the value negative and blocking that thread. Once all of the
    //threads are created, semSignal() will be called to release each of the threads
     sem_init(&threadCreation,0,0);


     sem_init(&tester,0,1);


    //Spawn all of the opener threads, 1 for each valve
    pthread_t threads[T_Valve_Numbers];
    int check;

    //Loop starts at 1 instead of the standard 0 so that numbering of valves
    //is somewhat more logical. 
    for(int i =1; i <= T_Valve_Numbers;i++)
    {
        cout<<"Creating thread: "<<i<<endl;
        check=pthread_create(&threads[i], NULL, Valve_Handler,(void*)&i);
        if(check)
        {
            cout <<"Couldn't create thread "<<i<<" Error: "<<check<<endl;
            exit(-1);
        }
    }

    //Release all of the blocked threads now that they have all been created
    for(int i =1; i<=T_Valve_Numbers;i++)
    {
        sem_post(&threadCreation);
    }

    //Make the main process wait for all the threads before terminating
    for(int i =1; i<=T_Valve_Numbers;i++)
    {
        pthread_join(threads[i],NULL);
    }
    return 0;



}   
    void* Valve_Handler(void* threadNumArg)
    {   
        int threadNum = *((int *)threadNumArg);
        sem_wait(&threadCreation);//Blocks the thread until all are spawned

        sem_wait(&tester);
        cout<<"I'm thread "<<threadNum<<endl;
        sem_post(&tester);
    }

当 T_Valve_Numbers = 10 时，一些示例输出为:

Creating thread: 1
Creating thread: 2
Creating thread: 3
Creating thread: 4
Creating thread: 5
Creating thread: 6
Creating thread: 7
Creating thread: 8
Creating thread: 9
Creating thread: 10
I'm thread 11   //Where is 11 coming from?
I'm thread 8
I'm thread 3
I'm thread 4
I'm thread 10
I'm thread 9
I'm thread 7
I'm thread 3
I'm thread 6
I'm thread 6   //How do I have 2 6's?

或

Creating thread: 1
Creating thread: 2
Creating thread: 3
Creating thread: 4
Creating thread: 5
Creating thread: 6
Creating thread: 7
Creating thread: 8
Creating thread: 9
Creating thread: 10
I'm thread 11
I'm thread 8
I'm thread 8
I'm thread 4
I'm thread 4
I'm thread 8
I'm thread 10
I'm thread 3
I'm thread 9
I'm thread 8 //Now '8' showed up 3 times

“我是线程...”打印了 10 次，所以看起来我的信号量正在让所有线程通过。我只是不确定为什么他们的线程号搞砸了。

Answer 1

check=pthread_create(&threads[i], NULL, Valve_Handler,(void*)&i);
                                                             ^^

您将 i 的地址传递给线程启动函数。 i 在主循环中一直在变化，与线程函数不同步。一旦线程函数开始实际取消引用该指针，您就不知道 i 的值是多少。

如果这是您唯一需要传递的东西，则传递一个实际的整数而不是指向局部变量的指针。否则，创建一个包含所有参数的简单 struct，构建这些参数的数组(每个线程一个)并向每个线程传递一个指向其自身元素的指针。

示例:(假设您的线程索引永远不会溢出 int)

#include <stdint.h> // for intptr_t

...
check = pthread_create(..., (void*)(intptr_t)i);
...

int threadNum = (intptr_t)threadNumArg;

更好/更灵活/不需要可能不存在的 intprt_t 示例:

struct thread_args {
  int thread_index;
  int thread_color;
  // ...
}

// ...

struct thread_args args[T_Valve_Numbers];
for (int i=0; i<T_Valve_Numbers; i++) {
  args[i].thread_index = i;
  args[i].thread_color = ...;
}

// ...

check = pthread_create(..., &(args[i-1])); // or loop from 0, less surprising

尽管如此，请注意:线程参数数组需要至少在线程使用它时保持事件状态。在某些情况下，您最好为每个结构进行动态分配，将该指针(及其所有权)传递给线程函数(尤其是如果您要分离线程而不是加入它们)。

如果您打算在某个时候加入线程，请像保留 pthread_t 结构一样保留这些参数。 (如果您在同一个函数中创建和加入，堆栈通常就可以了。)